All Derfs are Enajs. One-third of all Enajs are Derfs. Half of all Sivads are Enajs. One Sivad is a Derf. Eight Sivads are Enajs. The number of Enajs is 90. How many Enajs are neither Derfs nor Sivads?

God knows everything and always replies the truth. The devil knows
everything, but always lies. The third person's answers are completely
useless and could be right or wrong. Using three questions, determine who
is who.

If you have a mathematical system with several axioms (call them A, B,
C, D, E and F), is it possible to have two proofs of a theorem in this
system where one proof uses only axioms A, B, and C and the other
proof uses only axioms D, E, and F? In other words is it possible for
two proofs to use no common axioms? Or are all proofs of that theorem
really based on the same set of axioms?

An interesting logic puzzle about determining a birthday leads to a
discussion about interpretation, logic, and seeming confusion when one
of the logic statements is rewritten in a different but equivalent form.

0 percent probability should mean that it is impossible for a rational
number to be chosen from the set of real numbers, but obviously this
isn't the case. How can it be 0 percent probability but not impossible?